In the "Overseas Eastern Longitude", there is such a story: the emperor ordered the erection of the sea to measure the length of the east and west. Zhuhai is a very good walker. According to the description of "Huainanzi Topography", Yu sent Taizhangbu to measure the length of the east and west, and the vertical step to measure the length of the north and south. Scholars explain that "Taizhang" refers to the ancestral deed of the Shang people, while "Zhuhai" refers to the first prince of the Shang family. This coincides with the record of "merchant geodesy" mentioned in the Warring States Bamboo Book "Bao Xun". Dayu's measurements of east-west and north-south lengths were done by merchants. This is consistent with the description in the poem "Poems, Shang Song, Long Hair", which mentions that Dayu demarcated the borders of Kyushu. "The Biography of Mao's Poems" explains that "Jiuwei" refers to the boundaries of Kyushu. "Shang Song: Long Hair" records the situation of Chengyu's exploration and planning of Kyushu again. It is mentioned in Huainanzi Topographic that the distance between east and west is 28,000 li and the distance between north and south is 26,000 li in the whole country, which is consistent with the description in the Classic of Mountains and Seas. This data is calculated by the number of steps. However, could the ancients have really been able to walk such long distances?According to historical records, Dayu did go to the "four poles" when he controlled the water and demarcated Kyushu, but he did not specifically measure the length. "Shangshu Yu Gong" describes the boundary range of Kyushu after the success of Yu's water control.
Yu Gong records that the border of Kyushu extends to the sea in the east, quicksand in the west, and Shuonan to the four seas. He Nu's team of researchers focused on the "Gui Biao" unearthed in Tao Temple. According to their research, this ancient observatory is about 4,700 years old, extending to the coast in the east, quicksand in the west, and Lake Baikal in the north. While the ancients may have spent many hours measuring on foot, they were more than capable of doing it. It can be said that a "ten" shape has been formed here, which is what the ancients called the "four directions and five places", that is, east, west, south, north and middle. This is exactly what the ancients gained through "observing the time" or "measuring the shadow of the pole". The earliest known archaeological evidence of the practice of "pole photography" by Chinese ancestors shows that this activity began about seven or eight thousand years ago. The "rod" used by the ancients to measure the change of the sun's shadow is also known as "table", "gui" or "sundial". Through the "pole shadow measurement", the ancestors accurately grasped the changes of the sun shadow at the four key time points of the "spring equinox, autumn equinox, summer solstice, and winter solstice" about 7,000 years ago, and measured the four accurate directions of the southeast, northwest, and northwest, realizing the unity of "time and space".
The "+" sign can be seen on the artifacts unearthed 7,000 years ago, which was later called the "two ropes", that is, the meridian and the unitary line. The ancients observed that in the starry sky at night, the most distant and elusive constellation in the north is the "Beidou", which they regarded as the "celestial pole", believing it to be the very center of the sky and the highest position. Based on this observation, the ancestors began to search for and measure the corresponding "middle of the earth" on the earth. Ancient books such as "Shangshu Yaodian" and "Baoxun" of the Warring States Bamboo Book recorded the deeds of Di Yao, Di Shun and Shangjia Micro Survey. The ancients believed that the "Heavenly Pole" was the abode of the "Emperor of Heaven" or "God", and that the kings of the world should also "rule in the middle". This idea is recorded in many texts, the famous example of which is "Zhaizi Zhongor" in the inscription of the bronze Yi vessel "He Zun". The "or" here can be understood as "domain" or "country". This idea and its record also became the origin of the later term "China". In ancient times, the summer solstice was an important holiday. To determine the precise time of the summer solstice, the ancients used a variety of methods. Among them, the most widely used methods are "geodetic" and "guitable". "Geodesic" is the determination of the time of the summer solstice by observing the length of the sun's shadow.
When the length of the shadow was one foot five inches or one foot six inches, the ancients called it "in the middle of the earth". In addition, for every inch added or decreased, the corresponding distance increased or decreased by thousands of miles. These data were personally measured and confirmed by the ancients, so they are very reliable. Based on these data, the pre-Qin period of the Zhou Sutra established the relevant mathematical formulas by using mathematical formulas such as the "Pythagorean theorem". These formulas made important contributions to the development of ancient astronomy and mathematics, and were widely used by later scholars in astronomy, mathematics, geography and other fields.